The present invention relates to a mechanism for returning to a neutral position of an actuator. The present invention relates to a mechanism for returning a servo actuator, used in equipment, such as an aircraft, which especially requires high safety, to its neutral position when electric system for input and output signals breaks down.
In general, an aircraft requires high safety. In order to meet with this requirements, an aircraft is generally provided with two or more control systems for one object to be controlled so that a flight can be continued by using the remaining control system even when one of the control systems breaks down.
Even in this case, if the broken down control system maintains the condition under which the breakdown has occurred, the operation by the remaining control system may be obstructed, and accordingly, the safety of flight may be endangered. Thus, in such a case, it is necessary for the broken down control system to be returned to its neutral position wherein the flight is not adversely influenced upon by the broken down control system.
Mechanisms for returning a servo actuator, which controls wing, to its neutral position upon its breakdown have been conventionally proposed. Such conventional mechanisms for returning to the neutral position are roughly classified into two basic groups.
In the first group, as disclosed in Japanese Patent Laid-open No. Sho 63-297802, No. Hei 1-266308, and No. Hei 2-113101, the servo control valve is forcedly actuated upon breakdown so as to return the actuator to its neutral position.
In the second group, as disclosed in Japanese Patent Laid-open No. 63-308204, the passage in the servo control valve is closed upon breakdown, and a centering valve which is additionally disposed is actuated so as to return the actuator to its neutral position.
However, these conventional mechanisms for returning neutral position have the following disadvantages. More specifically, in the mechanisms belonging to the first group, the servo control valves per se have to be specially designed and accordingly, commercially available servo control valves cannot be used therefor. Thus, their manufacturing cost is expensive.
Contrary to this, in the mechanisms belonging to the second group, a directional control valve or a stop valve has to be disposed between the mechanism for returning to neutral position and the actuator. Accordingly, the hydraulic circuit becomes complicated and the size of the mechanism becomes large, and accordingly, their manufacturing cost is also expensive.
In addition, it is necessary for the mechanisms of the second group to be provided with a plurality of restrictor check valves so as to prevent a runaway of the actuator, i.e., a condition wherein the piston of the actuator moves to the operating end at a stretch, or an uncontrollable condition of the actuator upon switching from a normal condition to a breakdown condition or from a breakdown condition to a normal condition. However, in actual usage, although the runaway speed can be reduced, occurrence of such a runaway or an uncontrollable condition cannot be completely prevented.
Further, when, for example, the mechanism is applied to a mechanism for operating a steering flap of an airplane, it is preferred from an operational point of view that it initially moves slowly upon return to the neutral position, it gradually increases its speed and then it stops at the neutral position. However, because of the construction as described above, the moving speed of the actuator is slow since the restrictor check valve is disposed in the return oil passage from the centering valve, and accordingly, the actuator cannot be returned quickly to the neutral position.